1. Field of the Invention
This invention relates to a fuel control system for an engine, and more particularly to a fuel control system for an engine in which the amount of fuel to be fed to the engine is increased upon acceleration.
2. Description of the Prior Art
There has been known a fuel control system for an engine in which the amount of fuel to be injected is determined on the basis of the amount of intake air detected by an airflow meter, and fuel is injected for each cylinder or for all the cylinders at a predetermined timing.
In the fuel control system disclosed, for instance, in Japanese Patent Publication No. 60(1985)17939, additional fuel is injected out of the timing when the engine is being accelerated.
In order to effect such acceleration fuel increase, it is necessary to determine that the engine is being accelerated. Conventionally it is determined that the engine is being accelerated when the rate of change (dQa/dt) of the amount of intake air Qa as detected by the airflow meter exceeds a reference threshold value .alpha..sub.o.
When a hot-wire type airflow meter or a film type airflow meter is used for detecting the amount of intake air, the output of the airflow meter pulsates. In order to compensate for influence of the pulsation of the output of the airflow meter, it has been proposed to smooth the output of the airflow meter and to determine whether the engine is being accelerated by the use of the difference between the actual amount of intake air and the smoothed value of the amount of intake air.
As shown in FIG. 6, the output of the hot-wire type airflow meter Qa pulsates accompanied by overshoot upon acceleration of the engine and the rate of change with time of the output of the airflow meter is greatly affected by the pulsation. Accordingly when the reference threshold value .alpha..sub.o is relatively small, misjudgement can be caused due to the pulsation. Accordingly, conventionally, the reference threshold value .alpha..sub.o is set to be large. As a result, acceleration of the engine is detected and the acceleration fuel increase is performed after a substantial delay from the time the throttle valve begins to be opened, whereby acceleration response is lowered and acceleration shock is caused.
Further in the case of acceleration from a state where the amount of intake air is small, the amplitude of the pulsation of the output of the airflow meter is large, and accordingly, it is preferred that the reference threshold value .alpha..sub.o be set to be larger. On the other hand, in the case of acceleration from a state where the amount of intake air is large, the amplitude of the pulsation of the output of the airflow meter is small and there is less possibility of misjudgement, and accordingly it is preferred that the reference threshold value .alpha..sub.o be set to be small.
However, since the value of the reference threshold value .alpha..sub.0 is conventionally fixed irrespective of the amount of intake air, acceleration response is lowered though misjudgement can be avoided when the reference threshold value .alpha..sub.o is too large while misjudgement occurs frequently due to the pulsation of the output of the airflow meter when the reference threshold value .alpha..sub.o is too small.